Epoxy resins, especially those prepared from bisphenol A and epichlorohydrin, are known base materials for the production of high-quality casting resins and coating compositions. These aromatic epoxy resins, cured using amines, generally possess not only good chemical and solvent resistance but also good adhesion to numerous substrates. Bisphenol A epoxy resins which can be processed without the use of a solvent and have a very low viscosity are accorded particular importance, for example for the protection and renovation of concrete structures, as concrete additives such as for epoxide cement concrete (ECC field), for elastic adhesive systems in various fields of application, and for the coating of a wide variety of substrates. When amines are used, the epoxy resins can generally be cured at ambient temperature.
The use of Mannich bases of polyamines, phenols and formaldehyde is known for example, from U.S. Pat. No. 3,383,347, EP-A 0 042 617, U.S. Pat. No. 3,751,471, EP-A 0 068 263 and EP-A 0 253 339. With these products it is possible to produce, at low temperatures, thoroughly cured epoxy coatings which feature high pendulum hardnesses after a short drying time. The phenolic hydroxyl groups of the free or bonded phenolic structures are responsible for the accelerating effect.
One disadvantage of these systems is the more or less high content of free phenol, which greatly restricts the use of the hardeners due to environmental legislation and regulations.
EP-A 0 645 408 describes phenol-free hardeners having very good low-temperature properties. These epoxy systems cure rapidly, thus leaving the processor relatively little time for application. As described in Table 1 of EP-A 0 645 408 the pot life of such phenol-free hardeners is from 15 to 20 minutes.
There is a desire for systems having good low-temperature properties, i.e., those capable of rapid and complete drying at temperatures below 10.degree. C. so as to form coatings of high pendulum hardness, but which permit a relatively long processing time. At the same time, such hardeners are required to have a relatively low viscosity, i.e., below 5000 mPa.multidot.s at 25.degree. C., and to form a coating with a surface that is free from clouding and is insensitive to water.